Candy, I teach - and have taught - many, many children like your DS, both in Y3 and in the couple of years above.
I do not tell their parents that i remind them every 5 minutes to concentrate and sit still - it is my job to do that, and I do it because it is my job in order for those children to learn. I reseat everyone in the class according to the best position for these children not to disrupt others,. i deploy TAs, and plan enormous amounts of intervention and specify exactly what happens to them on my daily plans.
But i don't tell parents about this particularly, because it's normal. it's normal to differentiate for behaviour as well as for learning. I would no more tell the parent about the differentiation that i do for their child daily than I tell the parents of my most able pupil about the detail of the differentiation I do for them. At parents' evenings, and in reports, I say 'he / she is improving; their concentration span is increasing; with support and reminders they are getting more work done, which is great'.
But it doesn't mean that those children have no impact on the rest of the class, that other members of the class wouldn't form their own conclusions from the specific seating plans that separate particular children; the seating of the TA; the constant verbal reminders; the additional resources that that child will have on their desk / the carpet to fiddle with.
If those children weren't there, the rest of the class would have a different experience. i don't resent those children, any more than I resent the very able children, or the emotionally needy children, or the hungry children who need breakfast, but they change the experience of the class and they take time out of the day that could be used elsewhere with specific impact on other children - because however hard i try, my TA and cannot be everywhere, all the time.
it is low-level misbehaviour. It will pass, probably, or will be increasingly contained as the classroom becomes more structured - or it won't pass, and you will start to hear more about it because it will become less and less age appropriate and in secondary such behaviour is generally less tolerated than in primary, where we are still educating children about being in school. But it ISN'T negligible, and it DOES have an impact.